Conventional image communication methods include not only facsimile transmission (hereinafter simply referred to as the “FAX”) using a public telephone line (PSTN), but also FAX transmission using a high-speed network, such as an IP network. The method of FAX transmission using the IP network includes two types. One type is used in T.30 in-band communication using an ITU-T Recommendation T.30 protocol. This method transmits a T.30 analog signal after converting the same to a digital signal. The T.30 analog signal is transmitted by deeming the same as a voice signal and the method is called T.30 in-band FAX communication. The other type is a transmission method defined by ITU-T Recommendation T.38. This method transmits a signal transmitted using the T.30 protocol directly as a digital signal, and the signal can be transmitted at a higher speed than by T.30 FAX transmission.
These two methods both use an SIP (session initiation protocol) as call control means, and realize Point-to-Point transmission on the IP network (PTL 1). Further, the IP network includes e.g. a digital subscriber line called the NGN (next generation network) provided by Nippon Telegraph and Telephone Corporation in Japan, and a private IP line using an IP private branch exchange (IP-PBX). Particularly, when the NGN is used, bandwidth guarantee, security management, etc. are performed, and hence, it is possible to transmit images at a high speed and with security.
Although the maximum transmission speed in the conventional PSTN is 33.6 kbps, the maximum 1 Mbps is guaranteed in the NGN, i.e. it is possible to transfer data approximately 30 times faster than in the PSTN. For an image communication apparatus that is capable of performing the above-described T.38 FAX communication and T.30 in-band FAX communication, there has been proposed a method of configuring an operation of the telephone system, on the IP network. This makes it unnecessary to connect to the PSTN, but enables the image communication apparatus to perform voice communication and FAX communication only by network connection. In this case, a handset and a child phone are also connected to the IP network similarly to the FAX communication, and voice data is exchanged by passing voice packets through the IP network. This is a so-called IP telephone. A method of encrypting voice packets on the IP telephone is based on ITU-T Recommendation G.711, and an RTP (real time protocol) is used as a transmission protocol. Also in such an image communication apparatus, it is necessary to support various reception modes of a conventional facsimile connected to an analog telephone line, and a remote reception function in a telephone operation in a manual reception mode. The above-mentioned various reception modes include an automatic reception mode, an answerphone (answering machine) connection mode, a FAX/TEL-switching mode, and a manual reception mode (PTL 2).
In the automatic reception mode, the image communication apparatus automatically starts FAX reception upon receipt of a call, without causing an operator to respond using a handset or a child phone connected to the apparatus.
The answerphone connection mode is selected when a telephone having an answerphone function is connected as a child phone. In the answerphone connection mode, the apparatus is automatically switched to FAX reception when an initial identification signal (CNG (calling tone) signal in a FAX procedure) is detected during execution of unattended recording by the answerphone.
In the FAX/TEL-switching mode, it is automatically determined whether a received voice call is for FAX transmission or voice communication. In the FAX/TEL-switching mode, if a CNG signal is detected after the call is once connected to the FAX function, FAX reception is automatically started, whereas if not, the handset or the child phone is caused to ring to answer the call.
The manual reception mode only supports voice communication, and differently from the answerphone connection mode and the FAX/TEL-switching mode, in the manual reception mode, the apparatus is not automatically switched to FAX reception. However, if the apparatus has the remote reception function, the apparatus can be switched to FAX reception according to an instruction from a user. The remote reception function is to have the apparatus main unit switched to FAX reception in response to an operation (predetermined dialing) from a telephone connected thereto.
In an image communication apparatus which is connected to the IP network and has the function of performing FAX communication (T.38 FAX communication and T.30 in-band FAX communication) and voice communication over the IP network, calls are received in the above-mentioned various kinds of reception modes. In this case, reception of a call for T.38 FAX communication and reception of a call for T.30 in-band FAX communication can be distinguished from each other based on a media attribute provided by a caller for use in the SIP procedure. Reception of a call is notified by a SIP INVITE signal, and the notification includes the media attribute (definition by “m=” included in the INVITE signal). Definition of “m=audio” indicates reception of a call for T.30 in-band communication or voice communication, and definition of “m=image” or “m=application” indicates reception of a call for T.38 FAX communication. Therefore, when a call for T.38 FAX communication is received, even when the reception mode is set to any mode, the apparatus can be automatically switched to T.38 FAX reception.
However, reception of a call for T.30 in-band communication and reception of a call for voice communication have the same media attribute, and hence cannot be distinguished from each other. For this reason, to switch the apparatus to FAX reception in the answerphone connection mode and the FAX/TEL-switching mode, it is necessary to determine whether or not the call is for FAX communication after connecting the call.
In a case where a call is received by using the conventional analog telephone line, if the reception mode is the answerphone connection mode or the FAX/TEL-switching mode, a voice signal on the line is transmitted to a FAX communication controller that controls FAX communication. The FAX communication controller is a so-called MODEM (modulator-demodulator).
Then, a CNG signal is identified using a filter function of the MODEM, and when the CNG signal is detected, the apparatus is automatically switched to FAX reception. Also in the image communication apparatus capable of performing T.30 in-band communication and voice communication over the IP network, CNG detection is similarly required.